Photosensitive compositions useful in preparing flexographic printing plates are well known in the art. These compositions generally comprise (1) an addition polymerizable, non-gaseous, ethylenically unsaturated monomer, (2) a photoinitiator or system activated by actinic radiation and (3) a thermoplastic, elastomeric polymeric binder comprising polymerized conjugated diene monomers. Flexographic printing elements can be made from these photosensitive compositions by solvent casting or by extruding, calendering or pressing at an elevated temperature of the photosensitive composition into the form of a layer or self-supporting sheet on a suitable casting wheel, belt or platen. The sheet can be permanently affixed to a suitable permanent substrate and, usually, a removable cover sheet is placed on top to protect the photosensitive composition.
Photopolymerizable elements and processes for their use in preparing relief printing plates are well known in the art: U.S. Pat. Nos. 2,760,863, 3,556,791, 3,798,035, 3,825,430 and 3,951,657.
Extrusion is the preferred method for making photosensitive elements using the photosensitive composition of the instant invention. Thus, it is important that the photosensitive composition of this invention extrudes well. It is equally important that this composition be developable in a semi-aqueous or aqueous systems and this composition should have storage stability.
Cold flow deleteriously affects storage stability in that an excessive amount of cold flow results in the material becoming unacceptable, i.e., edge fusion occurs which prevents unstacking of photopolymerizable plates without damaging the continuity of the photopolymerizable composition.
Generally, storability with minimization of cold flow is imparted by proper selection and formulation of the components of the photopolymerizable material.
Use of an additive component to change physical properties in a composition is well known. One example is using beads as filler in an organic polymer composition which may be optionally polymerizable as disclosed in U.S. Pat. No. 4,414,278. The polymeric beads are discrete, substantially nonswellable and crosslinked with an average diameter of 0.7 to 20 micrometers.
In contrast to the use of highly crosslinked, nonswellable beads in a compositon are swellable microgels. Microgel is a term originated in the paint industry and it includes crosslinked spherical polymer molecules of high molecular weight such as of the order of 10.sup.9 to 10.sup.10 with a particle size of 0.05 to 1 micron in diameter prepared by emulsion polymerization. The term microgel conventionally refers to a particle having some degree of crosslinking throughout the entire particle. Crosslinking renders these microgels insoluble but capable of swelling in strong solvent without destroying the crosslinked structure. The preparation and use of such microgels is disclosed in British Patent No. 967,051 and U.S. Pat. No. 3,895,082.
The term microgel as used herein is not used in its conventional sense, rather, the term refers to a particle having two domains--a crosslinked core and an aqueous processible non-crosslinked shell. The core should have less than 10% crosslinking and the shell consists of an acid-modified copolymer.
U.S. Pat. No. 4,726,877 teaches the use of microgels to replace all or part of the binder of photosensitive compositions to improve resistance to cold flow. The microgels discussed have some degree of crosslinking throughout the entire particle. Even the core shell microgels which are mentioned have a crosslinked shell as is evidenced by the presence of the crosslinker, butanediol diacrylate, in the shell. See, core shell microgels O and P discussed in Table 1 in column 7. There is no teaching or suggestion of using a core shell microgel which has less than 10% crosslinking in the core and an aqueous procesible shell consisting of an acid-modified copolymer. Nor is there any teaching of using an ethylenically unsaturated monomer which partitions in the shell.
Japanese Patent Application No. 52/116,301 discloses the use of a component described as microgel in a non-aqueous processible photosensitive composition use for molding plates. The microgel appears to be a core shell microgel which is described as a rubber type substance obtained by a graft polymerization of a vinyl monomer with a so-called rubber type base material having particle diameter of 0.01 to 10 microns. The microgel materials used are referred to as elastomers which by definition have glass transition temperatures below room temperature. It is believed that the microgel is incorporated to toughen the rubber type material. The principles underlying rubber toughening are discussed by Seymour Newman and C. B. Bucknall in "Polymer Blends", D. R. Paul, ed., New York, 1978, volume 2, pp. 63-127. The presence of small rubber particles dispersed in a matrix of a more brittle polymer promotes crazing in the matrix polymer on impact, delayilng the onset of crack formation. The dispersed phase must be incompatible with the matrix polymer to remain as a discrete phase, and the temperature of use must be above the rubber's glass transition temperature in order for it to function as a toughening agent. These two requirements are fulfilled by the microgels described in Japanese Patent Application No. 52/116,301.
A delustering coating compositon which contains fine particles is disclosed in U.S. Pat. No. 4,518,472. The composition is applied to a molded article to provide high abrasion resistance or scratch resistance. Such composition for coating is a liquid which differs from the solid films of the present invention.
U.S. Pat. No. 4,272,608 discloses photosensitive compositions comprising high molecular weight butadiene/acrylonitrile copolymer containing carboxyl groups cross-linked via selected metal ions. The butadiene/acrylonitrile copolymer components are compatible with, and can be soluble in, the ethylenically unsaturated compound in the proportions used. The copolymer components, which constitute the binder, and the ethylenically unsaturated compound are dispersed evenly within one another. Microgels are not used.
Compatibility is also mentioned in U.S. Pat. No. 4,323,636 which discloses that the thermoplastic-elastomeric block copolymer component should be compatible with and, preferably soluble in, the ethylenic component when used in a ratio range of 99:1 to about 1:1. It is mentioned that compatibility can be achieved when the ethylenic component is soluble in either of the component blocks of the block copolymer. Microgels are not used.